The increase of mobile communication has driven the already busy and complex radio signal spectrum to a new level to accommodate the massive transfer of voice and data desired by the market. To operate within such a saturated spectrum, tunable filters are desirable to maintain optimal signal strength values. In particular, RF MEMS can help to provide high linearity performance, high quality factor, and long lifetime, making such devices useful in a wide range of mobile handset applications.
Continued data communication increases over a wide range of bands in the radio spectrum are leading to increasing number of tuning events over the life of the handsets, which will require extended and improved tunable lifetime. One of the main reliability issues of conventional RF MEMS switches and other MEMS electrostatic actuators, however, is dielectric charging. The high electrostatic field needed to close the switch generates charge inside the dielectric layers, which can cause undesired drift of the switch characteristic. This effect can deteriorate the tuning performance of the device and can lead to an irrecoverable stiction in extreme cases.
The origin of such dielectric charging varies depending on the location, type of contact, and mechanism involved. For instance, charging can occur in the dielectric bulk through injection from the electrodes into the dielectric with relatively quick charge/discharge dynamics. In other situations, surface charging can depend on the type of contact used. Specifically, metal-dielectric contacts can produce surface charge by injection when the switch is actuated, whereas triboelectric effects can be the main mechanism when two dielectrics are in contact, where charge exchange can occur between dielectrics without injection from the metal due to both dielectric thicknesses. In addition, many parameters can contribute to surface dielectric charging, such as the dielectric materials used, the fabrication process, and the ambient operating conditions. Even in structures where bulk charging is minimal, surface charging by triboelectric effects can have an important role, being the key factor limiting the switch lifetime.
In any case, charging is generally a fast mechanism when the switch is actuated under high voltage, but discharge can be comparatively slow when the voltage is removed. Once the surface charge is created and the contact is broken after removing the applied voltage, the only way for the charge to dissipate is through diffusion across the thickness of the dielectric, which can be particularly slow because of the generally low bulk diffusivity in high-quality dielectric materials. As a result, surface charging can be particularly detrimental because once the charge is generated on the surface, it cannot be removed, and cumulative deterioration can take place.
Accordingly, it would be desirable for MEMS switch systems, devices, and methods for the construction thereof to be particularly designed to reduce the amount of charging that occurs.